Spherical-form measuring apparatuses for measuring sphericity, that is, form deviations from a perfect sphere, have been conventionally known. For example, Patent Literature 1 includes a spherical-form measuring apparatus for measuring the contour form of a sphere in a cross section cut through a given plane passing through the center of the sphere (referred to as the “form of a great circle” of the sphere). The spherical-form measuring apparatus in Patent Literature 1 uses a general roundness measuring machine and allows efficient sphericity measurement conforming to the Japanese Industrial Standard JIS B 1501 (2009) “Rolling bearings—Balls”.
Known as one spherical standard is a reference sphere that includes a stem part and a sphere part. The reference sphere is used in three-dimensional measuring machines and the like (refer to Patent Literature 2). Three-dimensional measuring machines and the like often refer to the contour form of the sphere part of the reference sphere in a cross section cut through a plane which passes through the center of the sphere part and is perpendicular to the central line of the stem part (referred to as the “form of the equatorial circle” of the sphere part). For example, Patent Literature 2 includes a description that five measuring points, including four points at equal intervals of 90 degrees on the equatorial line and one at the north pole, are often used for measuring a sphere, and the five measuring points are used to estimate four parameters representing the center coordinates and diameter of the sphere by the least-squares method. The form of the equatorial circle of the sphere part is often referred to in estimation of the center coordinates of the sphere part of the reference sphere. If the form of the equatorial circle can be measured in a process of measuring the sphericity of the reference sphere by using the spherical-form measuring apparatus in Patent Literature 1, the measurement efficiency can be improved further.
FIG. 8 is a perspective view showing the overall structure of a spherical-form measuring apparatus 9 in Patent Literature 1. FIG. 9 is an enlarged view of a reference sphere 20 held in a holding unit 90 of the measuring apparatus 9 and the vicinity thereof. The holding unit 90 is disposed on a turntable 5 of the measuring apparatus 9.
As shown in FIG. 8, a slider 3 which can move in the Z direction is provided on a column 2 standing upright on a base 8 of the measuring apparatus 9. The slider 3 has a crank arm 4 which can move in the X direction. A probe 6 is secured to one end of the arm 4. The probe 6 has a stylus that is held so as to swing freely and detects the amount of displacement of the tip of the stylus. The slider 3 and the arm 4 can translate the probe 6 to a position where the tip of the stylus comes into contact with a sphere part 22 of a reference sphere 20.
The holding unit 90 includes a main part 92 and an inclined rotational holding part 94, as shown in FIG. 9. The main part 92 has an inclined face 93, and the inclined rotational holding part 94 is disposed rotatably on the inclined face 93. The surface of the inclined rotational holding part 94 has a hole formed along the rotational axis. A stem part 24 of the reference sphere 20 is held at an angle with respect to the surface of the table with its end inserted into the hole. When the inclined rotational holding part 94 rotates around its rotational axis while holding the reference sphere 20, the sphere part 22 of the reference sphere 20 rotates around the central line of the stem part 24. Suppose here that the angle formed between the central line of the stem part 24 of the held reference sphere 20 and the surface of the table is θ. The angle θ is set within a range of ±5 degrees with respect to an angle which has a sine value of 1/√3 (1 divided by the square root of 3). In other words, the attitude of the inclined rotational holding part 94 with respect to the holding unit 90 is determined to obtain the angle θ.
With alignment controls 7 on the turntable 5, the position and attitude of the top of the turntable 5 can be adjusted so that the center of the sphere part 22 of the reference sphere 20 is aligned to the rotational axis of the turntable 5.
When the turntable 5 turns the holding unit 90 with the tip of the probe 6 kept in contact with the sphere part 22, the contour form of a great circle C1 of the sphere part 22 is detected as displacements of the stylus, and the form data of the great circle C1 can be obtained in accordance with the displacements. The sphere part 22 can be turned in steps of 120 degrees around the central line of the stem part 24 of the reference sphere 20. By measuring the contour form at each rotation step, the contour forms of the sphere part 22 in cross sections cut through three planes which pass through the center of the sphere part 22 and which are roughly perpendicular to one another, that is, the forms of three great circles, can be measured.